A. Field of the Invention
The present invention relates to digital signal transmission and reception, and more particularly, to a system for and method of detecting horizontal and frame synchronization in a received digital signal.
B. Description of Related Art
The background of the present invention is described herein in the context of pay television systems, such as cable television and direct broadcast satellite (DBS) systems, that distribute a variety of program services to subscribers, but the invention is by no means limited thereto except as expressly set forth in the accompanying claims.
In the pay television industry, "programmers" produce programs for distribution to various remote locations. A "program" may consist of video, audio and other related services, such as closed-captioning and teletext services. A single programmer may wish to supply many programs and services. Typically, a programmer will supply these services via satellite to individual subscribers (i.e., DBS subscribers and/or cable television operators). In the case of cable television operators, the services transmitted via satellite are received at the operator's "cable head-end" installations. A cable operator typically receives programs and other services from many programmers and then selects the programs/services it wishes to distribute to its subscribers. In addition, a cable operator may insert locally produced services at the cable-head end. The selected services and locally produced services are then transmitted to the individual subscribers via a coaxial cable distribution network. In the case of DBS subscribers, each subscriber is capable of receiving a satellite down-link from the programmers directly.
In the past, pay television systems, including cable and DBS systems, have operated in the analog domain. Recently, however, the pay television industry has begun to move toward all digital systems where prior to transmission, all analog signals are converted to digital signals. Digital signal transmission offers the advantage that digital data can be processed at both the transmission and reception ends to improve picture quality. In addition, digital data compression techniques have been developed that achieve high signal compression ratios.
In addition, there is a growing trend toward a merger of the previously separate technologies of telecommunications including voice and data telecommunications and television including satellite, broadcast and cable television. There has emerged an increased interest in developing adaptable transmission systems capable of handling any one or more of a collection or plurality of such services. The primary media investigated for providing such services to date comprise, for example, coaxial cable, land-based microwave, so-called cellular radio, broadcast FM, broadcast satellite and optical fiber, to name a few.
Each media has its own characteristics. For example, comparing cable and satellite for digital data transmission, cable tends to have a medium error rate, but, when errors appear, the errors come in long bursts. Satellite as a media has a pretty poor error rate, primarily due to the requisite weak signal power, and hence, low signal to noise ratio. In satellite, then, the poor error rate is specially corrected utilizing such techniques as convolutional error correctors, not required in a cable environment.
In copending U.S. application Ser. No. 07/968,846, filed Oct. 30, 1992 and entitled "System and Method for Transmitting a Plurality of Digital Services," there is described an encoder for generating a multiplexed data stream carrying services to remote locations via, for example, a satellite or a cable distribution network. The generated data stream comprises a continuous sequence of frames, each frame comprising two fields, and each field comprising a plurality of lines. A first group of lines of a field defines a transport layer and a second group of lines defines a service data region. A feature of the disclosed scheme is the ability to dynamically vary the multiplexed data stream from field to field. A further feature of the disclosed scheme is that the data transmission rate of the multiplexed data stream is related to the frequency of known analog video formats, i.e. frame, field and horizontal line rates.
In copending U.S. application Ser. No. 07/970,918, filed Nov. 2, 1992, entitled "System and Method for Multiplexing a Plurality of Digital Program Services for Transmission to Remote Locations," there is described another system, this for multiplexing a plurality of digital program services comprising a collection of, for example, video, audio, teletext, closed-captioning and "other data" services. According to the disclosed scheme, a plurality of subframe data streams are generated, each having a transport layer region and a program data region. These subframe data streams are then multiplexed together into superframes having a transport layer region and a subframe data region.
While these disclosed transmission systems permit a variety of services to be transmitted over various media to remote locations, there remains a need to provide yet other alternative arrangements more particularly adapted to the wide variety of services that may be offered over various media and permit the end user at the remote location greater flexibility over the data content the user is ultimately enabled to receive. Moreover, such a system should be able to be easily adapted to transmit an increasing number of different services in an increasingly efficient manner, for example, utilizing the same or less bandwidth.
Since such services as high definition color television services, so-called "surround-sound" digital audio services, interactive transactional services for home-shopping, reservations, first-run as well as classic movie programming, software delivery, interactive games, alarm services, energy management and such all involve different bandwidths, data formats and such, there remains a need for flexibility in the overall structure provided for transmitting such services. Moreover, a user should not be presented with an overwhelming number of choices, but should be able, in a user-friendly manner, to select only those services which he is capable of receiving or wants to receive. Ideally, the user should be able to have access to an infinite variety of data services, selectable as he chooses, so that he may, for example, watch a first run movie in so-called high definition format accompanied by a "surround-sound" audio in the language of his choice and, at the same time, receive a facsimile or voice communication over the same media. If the user is equipment-limited, for example, to a standard resolution television and a telephone set, the user should be able to fashion the delivery of services to the equipment he owns.
Thus, there remains a need in such systems to efficiently and as inexpensively as possible recovery synchronization and clock information from a received digital signal so that the data representing the several multiplexed data services discussed above may be demultiplexed and distributed to the user's equipment.